1. Field of the Invention
This invention relates to a method for joining ceramics.
2. Description of the Prior Ar
In the prior art, both a "solid phase joining method", using diffusion reaction between solid phases, and a "brazing method", using function of liquid phase are known for joining a ceramics body with a metallic body or ceramics bodies with each other. However, the solid phase joining method is essentially superior relative to the properties of heat resistance and gas-tightness
For example, the solid phase joining method for joining the ceramics body of Al.sub.2 O.sub.3 with the metallic body of SUS405 (a kind of steel) is described in the journal "IONICS" (July, 1985, page 7). Next, this method will be described with reference to FIG. 1.
The joining surface of a ceramics body 4 of Al.sub.2 O.sub.3 to be joined, is polished with a 1 .mu.m-alumina abrasive material, and the joining surfaces of a metallic body 1 of SUS405, a Nb (niobium) sheet 3 and a Mo (molybdenum) sheet 2 are polished with a No. 800 abrasive paper. Then, they are stacked as shown in FIG. 1. The stacked body is heated and pressed at the temperature of 1300.degree. C. under the pressure of 100 MPa (mega Pascal) for thirty minutes by a hot-isostatic pressing machine (HIP).
The points to be considered in the joining of the ceramics body, are generally as follows:
(1) Relaxation of residual stress due to the thermal expansion coefficient difference between a ceramics body and a body to be joined with the ceramics body. PA0 (2) Control (suppression or promotion) of reaction in the surface boundary. PA0 (A) of preparing a ceramics body and another body to be joined with said ceramics body; (B) of interposing a layer of ultra-fine particles having smaller size than the surface roughnesses of the contact surfaces of said ceramics body and said other body, said layer having larger thickness than said surface roughnesses, and said ultra-fine particles being reactive with said ceramics body and said other body and forming a reaction product which has a strong bonding power to said ceramics body and said other body thereby forming a reaction product which has a strong bonding power to said ceramics body and said other body; and then (C) of pressing and heating the stacked composition of said ceramics body, other body and layer.
In the above prior art method, the Nb sheet 3 and the Mo sheet 2 are interposed between the Al.sub.2 O.sub.3 body 4 and the SUS 405 body 1 in order to relax the residual stress of the Al.sub.1 O.sub.3 body 4 due to the thermal expansion coefficient difference between the Al.sub.2 O.sub.3 body 4 and SUS405 body 1. The optimum material combination for the relaxation can be selected by the simulation of the internal stress using the finite element method. The thickness of the Nb sheet 3 and Mo sheet 2 are 0.5 mm, respectively.
Further in the above prior art method, the HIP machine is used for promoting the interfacial diffusion reaction. The contact area of the surface boundary should be sufficiently large, and the temperature should be sufficiently high in order that the interfacial diffusion reaction is promoted. In the usual solid-phase bonding method, one of the bodies to be joined with each other or both of them are deformed by the pressing operation so that the contact area between them is increased. In comparison with the joining of the both the metallic bodies, since the elastic coefficient of ceramics is very high, a remarkably high pressure is required for obtaining sufficient contact area of the ceramics body. The HIP machine can impart such a high pressure, and moreover isotropically. Accordingly, the metallic body is not remarkably deformed, but the contact area of the surface boundary between the ceramics body and the metallic body can be large.
However, the HIP machine is very expensive, and the method of using the HIP machine is not very efficient. The apparatus for joining is hard to be scaled-up or increased in size.